This invention relates to intrusion alarm systems and, more particularly, to alarm systems which are adapted to sense vibratory movement.
Intrusion alarms have been marketed for a substantial period of time and include a variety of sensing mechanisms which vary from extremely simple switches mounted across doorways or windows to extremely complex electronic apparatus for monitoring changes in magnetic reluctance or dielectric properties of an area to be protected. In the particular field of vehicle intrusion alarms, the vast majority of alarm systems are connected to sense the opening of vehicle doors only, particularly by monitoring the switches normally associated with each of the doors for illuminating a dome light. When any of these switches are closed, in addition to activating the dome light, the intrusion alarm systems activate an alarm mechanism such as the vehicle horn or a siren. In at least one prior art vehicle intrusion alarm system, a vibration sensitive element is included which incorporates a suspended weight which senses vibration. This prior art system uses a pair of contact points positioned on the suspension mechanism removed from the weight to sense vibration of the weight element. Since the movement of the suspension system adjacent the contact points is extremely slight, adjustment of the sensitivity of this prior art system is difficult. Furthermore, with such prior art systems, it has been common to operate the alarm mechanism directly from the contact points associated with the inertia sensing element, so that the points must carry a substantial current load. This often leads to eventual fusing of the contact points and a resulting malfunction of the alarm system.
In the general field of intrusion alarms, most alarm systems, regardless of the sensing mechanism, must be permanently attached and wired into the area or device being protected so that the alarm system itself substantially increases the cost of the protected item.